The present invention relates generally to surveillance systems, such as closed-circuit security or industrial inspection television systems, and more particularly to a novel system capable of permitting either daylight or enhanced night "vision" which operates with natural light without added illumination.
Image intensified television cameras suitable for use in closed circuit television (CCTV) systems are of two basic types. One involves the use of image orthicon television cameras utilizing lenses to focus the image onto the camera. To achieve better low-light sensitivity, such cameras are often supplemented by infrared illuminators. The second type consists of a lens to focus light onto the photocathode of a night-vision image intensifier tube, the image intensifier tube, and a direct coupled CCD device (charge coupled device) television camera or a transfer lens to focus the output of the intensified image onto the CCD input of the video camera. At the heart of such a CCTV system is the light amplifier image intensifier tube (usually referred to as a generation or Gen I, II or III type-device).
Image intensifier tubes (also called image enhancement tubes or simply image tubes) were first developed in the mid to late 1930's for military night vision applications. The early electro-optical low-light amplifiers were image converter infra-red tubes, also known as Gen 0 and Gen 1 night amplifier tubes. These were used successfully for many years. A successor to these tubes was the microchannel intensifier. It was a great improvement in size, cost and performance. A microchannel intensifier tube basically consists of a photo-sensitive cathode, a microchannel plate (MCP), and a phosphor output screen and means to create appropriate fields within the tube. The photocathode converts incoming photons representing an image to a corresponding spatially positioned stream of electrons. The electrons are accelerated to an MCP which intensifies the flow of electrons. At the output of the MCP the intensified electrons are accelerated again by another strong electric field to strike the luminescent phosphor screen whereat an enhanced visible image is created. The MCP consists of a two-dimensional array of miniature microchannel multipliers. A description of microchannel image intensifiers and the fabrication of microchannel plates can be found in "The Microchannel Image Intensifier," The Scientific American, Vol. 245 (November 1981) pp. 46-55 by Michael Lampton.
Microchannel image intensifiers are frequently employed today in applications requiring high amplification of extremely low-light levels. One obvious advantage of the current generation of microchannel image intensifiers is their light sensitivity which obviates the need for auxiliary irradiation either in the visible or near-infrared spectrum. They are particularly suited to night-time surveillance in military or police applications since they have high luminous gain, high image resolution and excellent low light sensitivity. In addition Gen III tubes are particularly sensitive in the near-infrared (NIR) spectrum, which makes them particularly useful in night time surveillance since night sky radiation is particularly high in the non-visible NIR region.
Image intensifier tubes and cameras have improved over the years, and today there are probably 50 different low light level camera systems made by a dozen or more manufacturers. Notwithstanding this there are drawbacks that limit use in modern image intenifier tubes even in the case of the latest generation units. Although relatively inexpensive when compared to earlier systems, there are many surveillance and security applications where the marketplace can not accept the $9,000 to $20,000 price tag of individual image intensifier TV cameras. In addition to the initial acquisition cost, the annualized replacement costs of image intensifier tubes or SIT camera tubes (silicon intensifier target) are inordinately high due to their restricted shelf life and even shorter operating lifetime. The operating lifetime of these tubes is dramatically reduced by exposure to high light levels. Also applications requiring continuous (24 hours/day) operation can shorten useful lifetime considerably. Gating of the high-voltage power supply has been used to limit the on time of the tube in certain applications, but there are other tradeoffs with such modifications. Compounding the replacement cost is the difficulty of replacing an image tube. It is often mechanically bonded to the optics assembly and TV camera requiring replacement of the entire system, or at the very least an involved and costly unsealing repair process.
This invention is directed to a novel video camera "attachment" which in its simplest form gives improved night surveillance and in its preferred form converts a normal day-light video camera to a day/night camera without requiring active or additional illuminators, IR or others, and which can be used with a color "day-time" monitor.
The invention may also be incorporated into a camera lens arrangements with automatic or manual iris or diaphram adjustment for day/night operation.
Another general object of this invention is to provide an image intensifier television camera design that minimizes the active operational time required of the image intensifier tube and thereby prolongs the life of the tube.